1. Field of the Invention
The invention relates to an electric power steering system including an auxiliary power supply.
2. Description of the Related Art
A conventional electric power steering system includes an auxiliary power supply connected in series with a main power supply. The electric power steering system is configured such that, when an assist motor that assists steering of a steering member consumes a large amount of electric power, for example, when a stationary steering (steering operation performed while a vehicle is not travelling) is performed, the assist motor is driven by electric power from the main power supply and the auxiliary power supply. Thus, the amount of electric power that the main power supply needs to discharge is restricted. The electric power steering system is configured such that, when the assist motor consumes only a small amount of electric power, for example, when the vehicle is driven without performing a stationary steering, the assist motor is driven only by the electric power supplied from the main power supply. The main power supply supplies electric power to electrical components. Japanese Patent Application Publication No. 2009-78743 (JP 2009-78743 A) describes an example of the configuration of such a conventional electric power steering system.
The voltage of the main power supply abruptly drops when a starter motor is driven in an engine start-up period. Thus, the voltage that is supplied from the main power supply to a microcomputer for driving electrical components may fall below the voltage required to drive the microcomputer. In this case, the microcomputer is initialized. This causes a possibility that the electrical components will malfunction. Note that such a possibility may be caused not only in the electrical components but also in a microcomputer for driving the assist motor.